1. Field of the Invention
The present invention relates to an optical scanning apparatus which deflection-scans an optical beam.
2. Description of the Related Art
Recently, digital copiers and printers have spread rapidly and various types of image forming apparatus have been put to practical use. A laser beam printer of an electrophotographic type, just one of various types, modulates a laser beam output from a semiconductor laser with an image signal, deflects the laser beam using a rotating polygonal mirror, and thereby scans a photosensitive drum. A latent image is formed on the photosensitive drum scanned with the laser beam and the latent image is developed into a toner image by a developing apparatus. The toner image is transferred from the photosensitive drum to paper via an intermediate transfer belt and then the toner image transferred onto the paper is fixed by heat treatment or the like.
In an image forming apparatus of such an electrophotographic type, a method which uses a main scanning synchronization signal (hereinafter referred to as a beam detection (BD) signal) is known as a method for controlling the rotational speed of a motor (hereinafter referred to as a polygonal motor) which rotationally drives a rotating polygonal mirror (Japanese Patent No. 3,526,136). The method detects the laser beam deflected by the rotating polygonal mirror, using an optical sensor (hereinafter referred to as a BD sensor) installed at a predetermined position, and outputs the detected signal as a BD signal. Next, the method frequency-divides the BD signal and controls the rotational speed of the polygonal motor to adjust the resulting period of the BD signal to a target period.
However, the above-described control method for controlling the rotational speed of the polygonal mirror has problems such as described below.
First, there are variations in laser beam scanning speed among mirror surfaces of the rotating polygonal mirror. Ideally, the rotating polygonal mirror has a regular polygonal shape. Actually, however, there are variations in surface geometry due to manufacturing errors. Since a reflection angle of the laser beam varies among the mirror surfaces due to the variations in surface geometry, the laser beam scanning speed varies among the mirror surfaces. Consequently, minute differences occur in magnification of scanning lines (length of scanning lines) in a scanning direction of the laser beam. That is, there are deviations in the magnification of formed scanning lines.
Moreover, when the rotational speed of the polygonal motor is controlled by supplying power to the polygonal motor, the rotational speed of the polygonal mirror may cause a repetition of minute fluctuations, which are visible when viewed closely. When the rotation of the polygonal motor is accelerated or decelerated, it takes a time equivalent to a few cycles of the BD signal from the time an acceleration/deceleration command is given until the rotational speed of the polygonal motor reaches a target speed.
To control the speed of the polygonal motor, the period of the BD signal is detected and power supply to the polygonal motor is increased or decreased based on an ACC signal or DEC signal output according to the detected period. In such a configuration, ideally power supply is increased or decreased until a voltage corresponding to the target speed of the polygonal motor is reached and then constant power is supplied to make the motor rotate stably at the target speed once the target speed is reached. Actually, however, charge supplied to the polygonal motor will leak. Consequently, even if the target speed is reached once, voltage of the polygonal motor will fall gradually, decreasing the rotational speed of the polygonal motor while a constant speed mode is maintained. This makes it necessary to supplement the charge periodically to make up for the leakage. In this way, when viewed closely, the rotational speed of the polygonal motor is accelerated and decelerated periodically.
When there are manufacturing errors of the polygonal mirror and variations in the rotational speed of the polygonal motor as described above, deviations in the magnification of the scanning lines attributable to the manufacturing errors of the polygonal mirror will increase unless the rotational speed of the polygonal motor is controlled with proper timing. That is, if the rotational speed of the polygonal motor is controlled with such timing that the rotational speed will be the lowest when the laser beam is deflected by the mirror surface on which the scanning line is the longest, the deviation in the magnification of the scanning lines in the laser scanning direction will be increased. The larger the deviation, the larger the variations in the scanning lines in the main scanning direction will be, resulting in degraded image quality. To improve the image quality, it is necessary to control the speed of the polygonal motor with optimum timing by taking into consideration accuracy in surface geometry of the polygonal mirror and control period of the polygonal motor.